The wettability of a material is dependent on both its physical and chemical characteristics. If a liquid spreads completely across the surface of a material and forms a film, the contact angle, θ, is close to 0 degrees (°). Such a surface may be said to be superhydrophilic. If the liquid beads on the surface, the surface is considered to be non-wettable by this specific liquid. For water, the substrate surface is considered to be hydrophobic if the contact angle is greater than 90°. Certain applications may require a hydrophobic coating with a high contact angle of at least 150°. These coatings may be said to be superhydrophobic.
Microfluidic systems on planar chips have gained popularity for handling miniscule volumes of liquids on the surface of open substrates. Open microfluidics offers a promising mode of digital microfluidics, which involves manipulating individual droplets without the need for dedicated components like microchannels, pumps, valves, sorters or mixers. Handling liquid on open substrates also minimizes the contact between the fluid and the channel walls, thus eliminating the risk of air-bubble clogging, fouling by debris and nonspecific surface adsorption of reagents. Besides, handling isolated droplets on the digital microfluidic platform minimizes cross-contamination between samples. However, achieving regular microfluidic tasks (e.g., sample drawing, metering, merging and dispensing) in a controlled fashion remains a challenge when using open microfluidic systems. Discrete microfluidic liquid transport technology has been achieved by electrowetting-on-dielectric (EWOD), optoelectrowetting (OEW), magnetic force, gravity, thermocapillarity, or acoustic vibrations. Surface wettability has played a supportive role in most of these applications by ensuring the desired droplet mobility and controllability. However, these active technologies require continuous power supply (or a desired orientation of the substrates in case of gravity-driven transport), and elaborate on-chip/off-the-chip interfacing arrangements (e.g., electrode array, permanent magnet assembly, sub-surface heating, etc.)—which for some applications are necessary—but they make their implementation more difficult.
Pumpless liquid transport technologies play an important role in the process of condensation. For example, condensation is not only ubiquitous in nature (rain from clouds), but it also plays a great role in large spectrum of engineering applications, starting from heat exchangers in power and process industries to fuel cells, from electronic thermal management to HVAC as well as water harvesting from the open atmosphere. From a thermodynamic perspective, achieving high condensation heat flux under minimum driving temperature difference (between the gas environment and the surface) is most desirable, because this improves the efficiency of energy conversion devices. Condensation heat transfer occurs in two primary modes, dropwise condensation (DWC) and filmwise condensation (FWC), the former offering an order of magnitude higher heat transfer coefficient (HTC) than the latter. HTC is a metric that, when maximized, allows optimal heat transfer operation, and thus maximum energy savings. However, achieving sustained DWC in engineering applications has remained an elusive task despite intense research for over half a century.
The overall performance of DWC depends on several factors, such as droplet nucleation density and rate, maximum size of departing droplets and rapid condensate drainage. It is desirable to design wettability patterned surface capable of controlling all the above three key factors (i.e., achieving optimal spatial nucleation, minimizing the departing droplet size and facilitating rapid drainage of condensate) necessary for enhancement of DWC. Still further, for low-cost microfluidics applications, a substrate-independent, yet straightforward surface preparation approach is desirable.